1. Field of the Invention
The present invention relates to a magnet temperature control device, and in particular to a magnet temperature control device for a permanent magnet magnetic resonance system.
2. Description of the Prior Art
Referring to FIG. 1, in a typical permanent magnet magnetic resonance system, magnets 20, pole plates 30 are respectively arranged in turn on upper and lower magnetic yokes 10. Stability is an important parameter for the magnetic field components of a permanent magnet magnetic resonance system, although there exists a plurality of external factors that affect the stability of a permanent magnet, in which the temperature stability is the most important one. However, the magnetic components for permanent magnet magnetic resonance, in particular the magnets 20 for generating the magnetic field are very sensitive to temperature changes. The reason is that the characteristics of the permanent magnet material change considerably with the temperature, thus the magnetic field excited by it has corresponding changes, and such changes are shown as fluctuation in magnetic field density and deterioration in its uniformity, and the abovementioned changes in the magnetic field will directly result in a decrease of imaging quality of the magnetic resonance system.
Therefore, in a magnetic resonance system, in particular a permanent magnet magnetic resonance system, the stability of the main magnetic field depends on the temperature stability of the magnet itself. In order to keep the temperature stability of the magnet, U.S. Pat. No. 5,652,517 discloses a technology for heating the magnets by using AC and DC heaters. In this patent, the AC and DC heaters are distributed on side faces of both the upper and lower magnetic yokes of the magnets, and a temperature probe is disposed close to the center of the upper pole plate. The DC heater is used only at the initial stage of temperature increase; under normal operation, the magnet temperature depends on the temperature probe and the DC heater for its control and adjustment.
PCT application PCT/GB99/02778 discloses another technology for maintaining the magnets' temperature stability; and in this patent application, since it is considered that heat is continuously dissipated to the surrounding environment from the surface of the magnet, the magnet temperature is kept constant by placing a DC heater on the surface of the magnet.
Japanese application JP3109042 discloses a technology for controlling the magnet temperature by using oil circulation; in this patent application, a loop for heating the magnet is formed by an oil groove, a heater, an oil pump and oil pipes wound on a magnet; and the output power of the heater is controlled by detecting the temperature difference between the inlet oil pipe and the outlet oil pipe, so as to accomplish the object of maintaining the magnet's temperature stability.
However, in the abovementioned various solutions in the prior art, the magnet temperature is adjusted by mounting a heating device or a constant temperature device on the surface of the magnet, without taking into the account the case that the magnet itself generates heat which needs to be dissipated. When heat is generated inside the magnet, by way of adjusting the heater on the surface of the magnet alone it can only ensure the surface temperature of the magnet to be constant, while the temperature change inside the magnet will keep oscillating for quite a long time and it cannot be eliminated, thus resulting in the fluctuation of magnetic density and deterioration of uniformity, and damaging the imaging quality of the magnetic resonance system.